The historian, Ibn Kathīr writes in al-Bidāyah (Dār Ibn Kathīr, 1431H) 8/13:

[In the year 42H] the Khārijites mobilised themselves, those whom ʿAlī had pardoned on the day of al-Nahrawān [after defeating them]. He had pardoned their wounded and had allowed them to keep their forces. However, when the killing of ʿAlī reached them, they invoked mercy upon his assassin, Ibn Muljam, and one of them said: ‘Allāh will not severe the hand which raised the sword over the rear of the head of ʿAlī.’ They began to praise Allāh for the killing of ʿAlī and then they resolved to attack the people. They concurred with each other on [embarking upon the duty of] ‘enjoining the good and prohibiting the evil’ as they claim.

This indicates that the Khārijites rarely abandon their doctrine and often lie in wait, lurking around, organising and waiting for the right opportunity to come out against Muslim societies. Despite the fact that ʿAlī spared the lives of many of them and let them go on their way, they rejoiced at his assassination, praised his murderer and mobilized themselves to attack people under the guise of ‘enjoining good’.

Abu Iyāḍ. 17 Muḥarram 1438 / 18 October 2016